One of the central questions in biology is to understand how cells communicate with their environment and the mechanism of signal transduction. Membranes play an essential role in this as well as many other cellular phenomena. The major objective of this research is to determine the role of lipid domains in activating protein kinase C. The development of fluorescence digital imaging microscopy has allowed the direct measurement of the lateral distribution of phospholipids in unilamellar vesicles and the red blood cell membrane. The results suggest a model of membrane structure with a heterogeneous distribution of lipid components that can change in response to a variety of agents such as calcium. It is proposed that lipid domains have an important role in a variety of membrane functions such as the activation of protein kinase C and signal transduction across membranes. Several external stimuli, such as hormones and growth factors, can cause the increased production of diacylglycerol in the plasma membrane and diacylglycerol acts as a second messenger to activate protein kinase C. Calcium and phosphatidylserine are also necessary for this process. Fluorescence digital imaging microscopy will be used to study the structure and organization of the membrane complex between protein kinase C, membrane lipids, calcium, and the substrates of protein kinase C phosphorylation under conditions that lead to enzyme activation. If the components are concentrated or ordered in a domain, this would greatly increase the reaction rate. This research also should help determine if factors that modulate the reaction interact directly with the enzyme or alter the nature of the membrane complex. Interactions between the two leaflets of the bilayer and the subsequent effect on the organization of the reaction components will be investigated as a mechanism for transmembrane signalling. It seems reasonable that many components of signal transduction pathways would show some organization into membrane domains since this would significantly increase the specificity and efficiency of the signal transduction process. It is proposed that the information obtained from these studies on protein kinase C will be of general significance for the lateral organization of membranes and signal transduction s well as for understanding the pathogenesis of membrane-related disease processes.